1. Field of the Invention
The present invention relates to an impedance adjustment circuit for adjusting terminal resistance and related method, and more particularly, to an impedance adjustment circuit evaluating the ratio of an off-chip resistor to a unit resistor by a successive approximation method to adjust the terminal resistance and related method.
2. Description of the Prior Art
For high-speed transmission interface, in order to optimize energy transmission efficiency and avoid unnecessary reflection during transmission, output resistance of a transmission terminal, input resistance of a reception terminal and characteristic resistance of transmission media, etc. in a signal transmission path are usually strictly limited to meet a certain standard value. For example, the normal characteristic resistance is 50 ohm or 75 ohm, and thus terminal resistance of a receiver circuit has to be designed to 50 ohm or 75 ohm as well.
However, on-chip resistors are usually affected by process variations to have ±15% error margin, and cannot achieve precise resistance value. Therefore, for the purpose of impedance match, the prior art usually utilizes an impedance matching circuit for adjusting the terminal resistance of the integrated circuit to match the characteristic resistance of the transmission media. In order to obtain more accurate terminal resistance, most impedance matching circuit fine tunes the resistance by precise off-chip resistors to meet the signal quality requirement, as disclosed in U.S. Pat. No. 7,368,936 and ROC Patent No. 538,602. However, the said patents both give a certain voltage on the off-chip resistor and consume enormous circuit area.